In communications systems using digital modulation schemes and transmitting information through a communication channel, information is transmitted through the communication channel by for example a signal at a predetermined carrier frequency which is modulated according to the information to be transmitted. Any reliably detectable change in or value of a physical parameter (e.g., amplitude, phase, frequency, etc.) of the signal sent by a transmitter of the communications system may reflect the information and, to retrieve the information, the change or value may be detected and demodulated by a receiver of the communications system. Thus, a conversion of the information from a binary or digital format to an analogue signal is typically required to determine the change or the value of the parameter before the signal can be transmitted.
The conversion of the information from the digital format to the analogue signal is typically performed in the transmitter in a converter unit that converts a digital value into a specific value out of a discrete set of M preset values (M being a positive integer) hereinafter referred to as the modulation signal set, for a modulated parameter, for example the phase, the amplitude or the frequency of the modulated signal.
Preceding the conversion, the digital value may be obtained e.g. by forming a symbol of N bits from a stream of bits in a manner that each symbol has a value out of a set of M discrete values (M=2N). It is typical that this conversion is performed by first mapping the digital value to a complex symbol or complex value with a real part and an imaginary part out of a set of M complex values, hereinafter referred to as the constellation set, using some predefined mapping scheme. The real part and the imaginary part are then used to define respectively an in-phase component and a quadrature component, i.e. the component with 90° degree phase shift with respect to the in-phase component, of the modulated signal transmitting the information. Thus, each point in the constellation set corresponds one to one to a single point in the modulation signal set.
Processing operations effected on the signal transmitted through the communication channel affect the signal and introduce noise. When the signal carrying the information reaches the receiver, the originally transmitted signal is corrupted by noise, which may depend for example upon the propagation conditions of the signal through the communication channel, noise introduced by the transmitter and by the receiver, etc. The received signal detected and demodulated by the receiver may thus have distorted in-phase and quadrature components relative to the in-phase and quadrature components of the originally transmitted signal. The distorted in-phase and quadrature components may cause the received complex symbols or complex values to be different compared to the originally transmitted complex symbols or complex values (which were selected out of the constellation set).
Digital communications systems typically have a unit which determines a link quality indicator (i.e. a measure for the quality of the channel) which is used by the transmitter and the receiver to optimally choose the transmission parameters to counter the amount of corruption generated. As link quality indicator typically a signal to noise ratio (SNR) of the received signal is used. A common measure of the SNR at the receiver is the so-called Error Vector Magnitude (EVM), which is the reciprocal of the SNR (or the negative if both the SNR and the EVM are expressed in decibel). The EVM is typically estimated by determining for the received complex symbol a most likely point out of the constellation set (hereinafter referred to as the original, nominal or assigned location) with a maximum likelihood detection algorithm. A distance (in the 2-dimensional complex space) of the actually received complex symbol or complex value from the most likely original nominal location is then determined, and the average distance forms a measure for the link quality.
There are alternatives disclosed in literature that seek to simplify the process of estimating the EVM, by for example limiting the use of the maximum likelihood detection algorithm. For example Chinese patent application publication CN101938450 discloses a method and a device for measuring a signal to noise ratio of a high order quadrature modulation. The method comprises the steps of transforming a symbol to obtain a corresponding position in a quadrature phase shift keying (QPSK) constellation diagram by performing QAM constellation transformation on the symbol in a high-order QAM constellation diagram; performing signal to noise ratio measurement on the symbol in the QPSK constellation diagram by a maximum likelihood method to obtain a first signal to noise ratio measurement value; and recovering the first SNR measurement value according to the order of the high-order QAM to obtain a second signal to noise ratio measurement value of the high-order QAM.
Further to determining the EVM using a maximum likelihood algorithm, other approaches are known in the art which make use of training sequences. For example patent application publication WO2012/044098 describes one of such approaches applied to a IEEE 802.11n system in which a preamble, known at the receiver side, is transmitted through the system to estimate the SNR without a communication channel estimation process. However prior art approaches as the one described in WO2012/044098 typically introduce a large overhead in the use of an available bandwidth of the communication channel.